Hydraulic cylinder for telescopic boom

ABSTRACT

A mobile crane has a boom comprising a plurality of telescopic hollow boom sections. Hydraulic operating cylinders for the boom sections are housed within the boom and each is provided with its own remotely operable electrically controlled main valve. Each cylinder comprises a housing and a rod therein which cooperate to define an extend chamber and a separate retract chamber. Passages in the rod connect these chambers to ports on the main control valve which is mounted on the rod. Hydraulic fluid is supplied to the valves by flexible hydraulic fluid hoses disposed between the side plates of adjacent boom sections and the hoses uncoil or coil in a frictionless manner as the sections are extended or retracted.

United States Patent 1 91 Johnston et al.

[ 1 Dec. 11, 1973 [54] HYDRAULIC CYLINDER FOR TELESCOPIC 3,610,433 10/1971 Milner et al. 91/411 R X O 3,666,125 5/l972 Gano et al. 214/141 [75] Inventors: Roger L. Johnston- Daniel Charles wiencek, both of edar Rapids Primary Examiner-Edgar W. Geoghegan Iowa Attorney-James E. Miles [73] Assi nee: Harnischfeger Corporation 8 Milwaukee, Wis. [57] ABSTRACT [22] Filed: Sept 18 1972 A mobile crane has a boom comprising a plurality of telescopic hollow boom sections. Hydraulic operating PP 239,987 cylinders for the boom sections are housed within the boom and each is provided with its own remotely op- [52] Cl 91/411 R 91/170 92/146 erable electrically controlled main valve. Each cylin 212/55 214/141 60/484 60/421 der comprises a housing and a rod therein which co- 51 1111.121 .FlSb i1/20 E621 3/00 Peme define extend chamber and a SePmate [58] Field of Search 91 411 R 414 170 retract chamber- Passages in the these 91/176. 92/146 5 212/46 chambers to ports on the main control valve which is DIG R mounted on the rod. Hydraulic fluid is supplied to the valves by flexible hydraulic fluid hoses disposed be- [56] References Cited tween the side plates of adjacent boom sections and the hoses uncoil or coil in a frictionless manner as the UNITED STATES PATENTS sections are extended or retracted. 2,643,515 6/1953 Harsch 9l/4 14 X 2,984,985 5/1961 MacMillin 60/471 X 9 Claims, 12 Drawing Figures ii v piggggii 679 1490 149c 5,5 67C 68 52 71 l '31 'i 67B 67A PAIENIEBIJEEI 1 ms SHEET 2 (IF 9 PAIENTEHIIEBIHQH 3,777, 529

SHHI 3 OF 9 HYDRAULIC CYLINDER FOR TELESCOPIC BOOM BACKGROUND OF THE INVENTION 1. Field of Use This invention relates generally to booms having a plurality of telescopic boom sections, such as are used on mobile cranes or the like. In particular it relates to improved hydraulic cylinder means for extending and retracting the boom sections.

2. Description of the Prior Art Present technology requires that mobile cranes and other equipment employing booms having telescopic boom sections be made increasingly larger and stronger so that the booms can be extended to greater lengths and can handle heavier loads. Consequently, as the overall extended length of the boom increases, larger and longer hoses are required to supply the hydraulic cylinders within the boom and disproportionately large hose reels are required to take up and pay out the hose as the boom operates. In some prior art telescopic booms, each linearly movable boom section is provided with its own hydraulic cylinder by which it is extended and retracted and the control valve for the cylinder is located remote from the cylinder. This arrangement requires a separate pair of flexible hydraulic fluid hoses connected between each valve and'its cylinder.

SUMMARY OF THE PRESENT INVENTION In accordance'with the present invention there is provided a mobile crane which has a boom comprising a plurality of linearly movable hollow telescopic boom sections. Hydraulic operating cylinders, one for each movable boom section, are housed within the boom and each is provided with its own remotely operable electrically controlled main valve which is physically mounted thereon. Hydraulic fluid is supplied to the valves by flexible hydraulic fluid hoses disposed between the side plates of adjacent boom sections in a looped arrangement so that the hoses uncoil orcoil in a frictionless manner as the sections are extended or retracted. More specifically, each cylinder comprises a housing, means on the housing to secure it to one boom section, a cylinder rod extending into the housing, and means on the rod to secure it to another boom section. The housing and rod cooperate to define a retraction chamber and a separate expansion chamber. The remotely operable control valve is mounted on the end of the rod and has two fluid input ports and two fluid output ports. A first passage in the rod interconnects the other fluid outlet port and the retraction chamber. There is provided a plurality of pairs of hoses for the valves, each pair of hoses connected between the valves of adjacent cylinders; one hose in each pair being a fluid supply hose and the other hose in each pair being a fluid return hose. Electric control wires for operating the electric valves are wrapped about the hoses and held in place thereon by heat-shrunk plastic tubing. The boom allows for a compact arrangement of boom operating components therein. Also, the hoses for the cylinders within the boom are fewer in number,

.and more simply and compactly arranged. Other objects and advantages will hereafter appear.

DRAWINGS FIG. 1 is a side view of a mobile cranehaving a telescopic boom in accordance with the invention;

FIG. 2 is an enlarged view of the interior of the boom shown in FIG. 1 with the boom sections shown in partially extended position;

FIG. 3 is another even further enlarged view of the interior of the boom shown in FIGS. 1 and 2 with the boom sections shown in fully retracted position;

FIG. 4 is an enlarged view of the base of the boom shown in FIG. 1;

FIG. 5 is an enlarged view, with some portions broken away, of one of the boom sections shown in FIGS. 1, 2 and 3;

FIG. 6 is a view of the boom section shown in FIG. 5, showing a portion of the rear end thereof and a portion in section on line 6-6 of FIG. 5;

FIG. 7 is an enlarged view of the boom shown in FIG.

3, showing a portion of the rear end thereof and a portion .in section on line 77 of FIG. 3;

FIG. 8 is an enlarged view, partly in cross section, of one of the hydraulic cylinders shown in FIGS. 2, 3, 7 and 9;

FIG. 9 is a schematic diagram of the hydraulic con trol system fo a boom in accordance with the invention;

FIG. 10 is an enlarged cross section view of an electrically operated hydraulic control valve shown in FIG.

FIG. 11 is a top plan view of taken on line llll of FIG. 7 showing the hose connections to the control valve for one of the cylinders; and

FIG. 12 is an enlarged cross section view of one of the hydraulic hoses in the boom showing electrical wires for the control valves secured thereto.

DESCRIPTION OF A PREFERRED EMBODIMENT FIG. 1 shows a mobile crane 10 comprising a chassis l1, ground wheels 12, an operators cab 13, a horizontally rotatable crane upper 14 mounted on the chassis, a telescopic boom 15 in accordance with the invention pivotably mounted on the crane upper, and a pair of boom hoist cylinders 16 (only one visible in FIG. 1) connected between the crane upper and the boom.

FIGS. 1, 2, 3 and 7 show that boom 15 comprises a plurality of telescopic boom sections, namely: the base section 20, an inner mid section 21, an intermediate mid section 22, an outer mid section 23, a fiy section 24, and a manual section 25. As FIGS. 1 and 4 show, the rear end of base section 20 has trunion mounts 26 on opposite sides which receive pins 27 that pivotably mount the base section on crane upper 14. The boom hoist cylinders 16 are located on opposite sides of base section 20 and are pivotably connected by pins 28 to support brackets 29 on crane upper 14. The rod ends of the boom hoist cylinders 16 are pivotably connected by pins 30 to connecting brackets 31 on opposite sides of base section 20. As FIG. 1 shows, the forward end of manual section has a working head 32 thereon which, for example, is provided with a pulley 33.

As FIGS. 2, 3 and 7 show, the boom 15 also comprises a plurality of hydraulic cylinders for extending and retracting the boom sections, namely: an inner mid section cylinder 41, an intermediate mid section cylinder 42, an outer mid section cylinder 43, a fly section cylinder 44 and a manual section cylinder 45. The cylinders operate the boom sections bearing the same name. FIG. 8 is an enlarged view, partly in cross section, of outer mid section cylinder 43 and a description thereof will suffice for the cylinders 41, 4 2, 44 and..45

which are identical thereto except as regards size. Cyl- 3 inder 43 comprises a hollow tubular housing 50 closed at one end by an end plate 51 within which a hollow tubular cylinder rod 52 is slideably mounted. The inner end of rod 52 is provided with a piston 53 having piston rings 54. Housing 50 and rod 52 cooperate to define a cylinder retract chamber 55 and the end of this chamber is closed off by sealing means 56 secured to housing 50 and having sealing rings 57. Cylinder rod 52 is provided with a cylinder extend chamber 58. Cylinder rod 52 is also provided with an internal passage 59 which is connected by a tube 60 to retract chamber 55. Cylinder rod 52 is further provided with an internal passage 61 which is connected to extend chamber 58. The ports of the passages 59 and 61 are connected by passages 62 and 63, respectively, in an adapter plate 64 on the end of rod 52 to ports 75 and 76, respectively, of an electrically operated cylinder control valve 67 attached to plate 64. Fluid pressure in passage 61 of rod 52 caused by extend operation of valve 67 causes fluid flow into extend chamber 58 and effects extend operation of cylinder 43.. Conversely, fluid pressure in passage 59 of rod 52 caused by retract operation of valve 67 causes fluid flow into retract chamber 55 and effects retract operation of cylinder 43.

Housing 50 of cylinder 43 is provided on its exterior with a trunnion 68 by which it is connected to a trunnion mount 69 on outer mid boom section 23. Cylinder rod 52 of cylinder 43 is provided on its exterior with a trunnion 70 by which it is connected to a trunnion mount 71 on boom section 22. As FIGS. 2 and 3 show, the trunnion 68 on the housing of each cylinder 41, 42, 43, 44 and 45 is connected to the trunnion mount 69 of that boom section which it operates. The other trunnion 70 on the rod end of the cylinders 41, 42, 43, 44 and 45 is connected to the trunnion mount 71 of the next adjacent boom section.

A hydraulic cylinder such as cylinder 43 offers several advantages over conventional hydraulic actuators or cylinders used in prior art cranes. For example, a conventional cylinder is normally connected and supported at each of its extreme ends (the outer rod end and the base end of the cylinder) to the boom sections associated therewith. Consequently, in extremely large cranes, as the cylinder is operated to extend the boom section, the two connection points of the cylinder move very far apart and the cylinder is subjected to buckling and bending forces which tend to increase the risk of mechanical failure of the cylinder and impose greater wear forces on internal moving components of the cylinder. In cylinder 43, however, the attaching and supporting trunnion 70 of cylinder rod 52 is located near the end of rod 52, whereas the attaching and supporting trunnion 68 of cylinder housing 50 is located near the midpoint of housing 50. Consequently, when cylinder 43 is in retracted or extended condition, the trunnions 68 and 70 are always relatively closer together than the attachment points of a conventional cylinder of comparable size would be. Thus, cylinder 43 is subjected to a reduced load and its buckling strength, especially when fully extended, is substantially increased. This factor is of extreme importance in larger cranes of the type described which have long and heavy boom sections and handle very large loads.

Referring again to FIG. 8, it is seen that cylinder 43 is provided with a conventional holding valve 72 which is mounted on plate 64 and operates to prevent undesired retraction of the extended cylinder until the holding valve is released (opened) in response to retract operation of the main control valve 67. Each cylinder 41, 42, 43, 44 and is provided with a similar holding valve.

The electrically operated main control valve 67 shown in FIG. 10 is an electrohydraulic proportional metering valve comprising a valve housing 73 containing a four-way sliding spool 74, two fluid control ports 75 and 76 which are connected through plate 64 to the passages 59 and 61, respectively, in rod 52 of cylinder 43. Valve 67 further comprises an electrical torque motor 80 and a nozzle flapper pilot stage for operating spool 74.

The torque motor 80 includes coils 81, polepieces 82, magnets 83 and an armature 84. The armature 84 is supported for limited movement by a flexure tube 85. The flexure tube 85 also provides a fluid seal between the hydraulic and electromagnetic portions of the valve.

A flapper 86 attaches to the center of the armature 84 and extends down, inside the flexure tube 85. A nozzle 87 is located on each side of the flapper 86 so that flapper motion vaies the nozzle openings. Pressurized hydraulic fluid is supplied to each nozzle through a filter 88 and inlet orifice 89. Differential pressures caused by flapper movement between the nozzles 87 are applied to the ends of the valve spool 74.

The four-way valve spool 74 directs flow from pressure supply chamber 92 to either control port 75 or 76 in an amount proportional to spool displacement. The spool 74 contains flow metering slots 90 in the control lands that are uncovered by spool motion. Spool movement deflects a feedback wire 91 that applies a torque to the armature/flapper. Spool detent springs 92 are provided to center the spool whenever hydraulic driving pressures are absent.

In operation, electrical current in the torque motor coils 81 causes either clockwise or counter-clockwise torque on the armature 84. This torque displaces the flapper 86 between the two nozzles 87. The differential nozzle flow moves the spool 74 to either the right or left. The spool 74 continues to move until the feedback torque counteracts the electromagnetic torque. At this point the armature/flapper is returned to center, so the spool 74 stops and remains displaced until the electrical input changes to a new level. Therefore, valve spool position is proportional to the electrical signal. The actual flow from the valve to the load will depend upon the load pressure.

A valve such as valve 67 is commerically available from Moog, Inc., Controls Division, Proner Airport, East Aurora, N.Y. 14052 and is disclosed in that companys Catalog 602.

FIGS. 5 and 6 show in detail the physical construction and configuration of intermediate mid boom section 22 which will now be described in detail; it being understood that the other boom sections 20, 21, 23, 24 and 25 are similar thereto. Section 22 comprises a top plate 100, a bottom plate 101, a right side plate 102, and a left side plate 103. Each of these four plates is fabricated of a solid plate or sheet of steel. The upper edges of the side plates 102 and 103 are joined by a continuous weld to the undersurface of the top plate 100. The lower edges of the side plates 102 and 103 are similarly joined by a continuous weld to the upper surface of the bottom plate 101. Side plate stiffeners 105 and 106 are edge-welded between the side plates 102 and 103, respectively, and the edges of the bottom plate 101. The side plate stiffeners 105 and 106 serve to increase the buckling strength of the side plates 102 and 103, respectively, by about a factor of five in the embodiment shown. The bottom plate 101 is substantially narrower than the top plate 100 (in a ratio of about five units to three units) and the angle a defined between a side plate and the bottom plate is an angle greater than but less, for example, than 120. Each side plate 102 and 103 is wider than the bottom plate 101 in a ratio of about 6 units to 3 units. Thus, the general-cross-sectional configuration of section 22 is that of a trapezoid or an inverted A-frame. In a practical embodiment of the invention, top plate is about 24.6 inches wide, 0.375 inches thick, and 383 inches long. Bottom plate 101 is about 13 inches wide, 0.500 inches thick, and 376 inches long. Each side plate 102 and 103 is about 33 inches wide, 0.150 inches thick, and 380 inches long. Each side plate stiffener 105 and 106 runs the length of section 22 and is about 6.375 inches wide and 0.250 inches thick.

It has been discovered through testing that boom section 22 affords the following advantages. First, the internal height of boomsection 22 is increased without an increase in other cross section properties, such as additional weight, thereby allowing more internal space for accommodating components such as the hydraulic cylinders. Second, the stiffener plates 105 and 106 change the end condition of the side plates 102 and 103 from simply supported to semi-fixed and thereby allow thinner side plates to be used in view of buckling considerations. Third, the relatively narrower bottom plate 101 allows a thinne'r bottom plate to be used in view of buckling considerations. Fourth, as FIG. 4 shows, the relative narrowness of the bottom of a boom section such as 22 (particularly boom base section 20) allows the storage position of boom 15 to be lower between the boom hoist cylinders 16 thereby affording lower overall height without increasing boom section width and without reducing the moment arm of the boom hoist cylinders. Fifth, the width of the top plate 100 of boom section 22 can be varied fordesired side strength purposes without affecting any of the above-listed advantages.

Referring again to FIGS. 5 and 6, boom section'22 further comprises a slide pad support plate 110 welded to the side plates 102 and 103 at the top rear end of the boom section. A pair of external slide plates 111 and 112 are welded to plate 110 and serve as bearing surfaces for the undersurface of the top plate 100 of the boom section 21 into which boom section 22 telescopes. Support plate 110 is braced by welded external braces 113 and 114 and by a welded horizontal inner brace 115 to which spaced apart vertical braces 116 are welded.

Rear stiffener plates such as 117 are provided at the rear end of boom section 22 an are welded to support plate 110 and an associated side plate 102 or 103.

A cylinder attachment assembly 71 and a trunnion mount 69, both hereinbefore referred to in connection with the description of cylinder 43, are provided at the outer rear end of boom section 22.

Boom section supports, such as support 120, are welded on the inner surface of the side plates 102 and 103 near the rear end of boom section 22. Each support 120 is rigidified and stabilized by a brace 121.

A pair of internal slide pads, such as pad 122, are mounted on a support 123, which is welded to the inner surfaces of the side plates 102 and 103 near the forward end of boom section 22. Support 123 is rigidified and strengthened by a welded horizontal brace 124 and welded vertical braces 125.

Polyethyle slide pads, such as 130, are mounted on steel back-up plates, such as 131, and extend through openings, such as 132, in the side plates 102 and 103 into the interior of boom section 22 where they frictionally engage the sides of boom section 23 which telescopes into boom section 22. Slide pads 135, similar to project outwardly on the sides of the forward end of boom section 22 for engagement with boom section 21 into which boom section 22 telescopes.

Referring now to FIGS. 2, 7 and 11, it is seen that the manual section cylinder 45, the fly section cylinder 44, and the outer mid section cylinder 43 fit within manual boom section 25 when boom 15 is in retracted condition. Intermediate mid section cylinder 42 and inner mid section cylinder 41 fit within intermediate mid boom section 22 and inner mid boom section 21, respectively. The main control valves 67A, 67B, 67 67C and 67D for the cylinders 41, 42, 43, 44 and 45, respectively, are supplied with hydraulic operating fluid from a pressurized fluid source, such as pumps 136 and 137 shown in FIG. 9, which is understood to be located on crane 10.

FIG. 11 is a top plan view of valve 67 on outer mid cylinder 43 and shows that hydraulic fluid T-fittings 140 and 141 are provided on opposite sides thereof. It is to be understood, for example, that fittings 140 and 141 are located on the extend side and the retract side, respectively, of valve 67. Furthermore, valve 67 is provided internally with a pressure compensated fluid control device 142 into which T-fitting 140 discharges. The fittings 140 and 141 on valve 67 are connected by flexible hoses 1493 and B,respectively, to the pumps 136 and 137 and to a fluid reservoir 147. Thus, hose 1498 is for fluid supply and hose 1508 for fluid return. The fittings 140 and 141 are also connected to fluid supply and fluid return hoses 149C and 150C, respectively, which are connected to the control valve 67C for the next cylinder 44. As FIG. 9 shows, the main control valves 67A, 67B, 67C and 67D for the other cylinders are each provided with fluid supply and return T-fittings designated respectively: 140A and 141A; 140B and 141B; 140C and 140C; 140D and 141D. These fittings are interconnected by flexible fluid supply and return hoses designated, respectively: 149A and 150A; 149B and 1508; 149C and 150C; 149D and 150D. Thus the main control valves are connected in parallel arrangement to the fluid supply and fluid return systems. As FIG. 9 also shows, the valves 67A, 67B, 67C, and 67D are provided with pressure compensated flow control devices 142A, 142B, 142C and 142D, respectively.

The hydraulic control system shown in FIG. 9 also includes a pressure responsive valve 156 for automatically connecting pump 137 to the system if system pressure indicates that more than one cylinder is being extended and additional fluid is required. The pumps 136 and 137 are understood to be engine-driven and operating continuously when crane 10 is in use. The hydraulic control system alsoincludes a pressure compensated flow control valve 157, a pressure switch 158, a relief valve 159, and solenoid controlled pressure relief valves 160 and 161.

As FIGS. 2 and 7 show, the flexible fluid supply and fluid return hoses such as 149C and 150C are physically located between the side plates of adjacent boom sections in a looped or coiled arrangement which enables the hoses to pay out or coil back upon themselves as the boom sections are extended or retracted, respectively. As FIG. 12 shows, the lower portion of each hose loop is supported or rests in support clips 155 which are secured at intervals on the outer surfaces of the boom side plates. It is to be noted that, although each hose flexes at the curved portion as the boom 15 is operated, the straight upper and lower segments of the hose do not slide on, rub or frictionally engage the sides of the adjacent boom sections but are stationary with respect thereto. Consequently, the hoses are not subjected to damaging abrasion or wear from this source but only to flexing action.

As hereinbefore explained, the main control valve 67, 67B, 67C and 67D are electrically operated in response to electric signals supplied to the torque motor thereon, such as torque motor 80 hereinbefore described in connection with valve 67. As FIG. 9 shows, each of the four electrically controlled main valves is connected by electrical conductors, generally designated 165, which extend from an electrical controller 166 on crane 10 to the valves. In practice only two conductors 165 are necessary for each main valve, although additional back-up conductors could be provided for each valve as a safety feature. Furthermore, it is to be understood that controller 166 is designed to enable actuation of each valve separately to allow any single boom section to be operated individually or to enable actuation of all valves simultaneously to allow all boom sections to be operated simultaneously and proportionately.

As FIG. 10 shows, the electrical conductors 165 for the valves are connected to a terminal block 167 provided on each valve. Between each valve, as FIG. 12 shows, the conductors 165 are twisted around a hydraulic hose, such as hose 149C, and secured in place thereon by means of a flexible heat-shrunk plastic sleeve or tube 168. Preferably, the conductors 165 take the form of braided insulated wires. This arrangement is advantageous in that the wires are securely held in place, not liable to damage as the boom sections move, and like the hoses, are not subject to frictional wear but only flexure.

The boom extension cylinders 45, 44, 43, 42 and 41 are sized progressively larger from top to bottom so that, although the working pressure required is least at the uppermost cylinder 45, the combined effect of required working pressure, induced load pressure and line pressure drops is such that pressure requirements as measured at the pump 136 or 137, or even at the boom base cylinder 41, are approximately equal for any given load, regardless of which cylinder or combination of cylinders are in use.

It is important, as FIG. 9 shows, that there is only one pressure compensated valve 67A, with auxiliary valves, such as 157, for providing unloading and main relief ca pabilities near the boom base, for directly controlling inner mid section cylinder 41. The other four valves 67B, 67, 67C and 67D are remote controlled electro hydraulic proportional metering valves mounted on cylinders in the boom. In practice, valve 67A, valve 157, valve 159 are all necessary interconnecting tubing are housed in a common valve housing (not shown).

The pressure compensating valve 157 functions by constantly measuring the difference between pump and cylinder pressure and dumping or unloading the excess pump flow as required to maintain the desirable differential. This would normally be impossible in conventional hydraulic control circuits for cranes when the most remote cylinder is 125 feet away, but if the system balance is such as is possible in the present invention so that the nearest cylinder pressure reading is the same as the furthest, then it is only necessary to measure and respond to the nearest pressure condition, and the unloading function of valve 157 serves the furthest cylinder equally as well as the nearest cylinder.

RESUME A mobile crane 10 has a boom 15 comprising a plurality of telescopic boom sections 20, 21, 22, 23, 24 and 25. Each boom section is fabricated of solid steel plates with the top plate being wider than the bottom plate 101 and the side plates 102 and 103 welded therebetween and sloped inwardly. Side plate stiffeners 105 and 106 welded between the bottom plate 101 and the side plates 102 and 103, respectively, run the length of the boom section. This configuration results in a boom section of minimum weight with a bottom plate 101 of relatively increased buckling strength and maximum interior space for boom operating components. Hydraulic operating cylinders 41, 42, 43, 44 and 45 for the boom sections 21, 22, 23, 24 and 25, respectively, are housed within the boom and each is provided with its own remotely operable electrically controlled main valve such as valve 67. Hydraulic fluid is supplied to and returned from the valves by flexible hydraulic fluid hoses such as hoses 149C and 150C disposed between the side plates of adjacent boom sections and the hoses uncoil or coil in a frictionless manner as the boom sections are extended or retracted. Electric control wires, such as 165, for operating the electric valves are wrapped about the hoses and held in place thereon by heat-shrunk plastic tubing 168.

Boom 15 in accordance with the present invention is relatively stronger than conventional booms of comparable weight and size, while at the same time enabling a more compact arrangement of boom operating components, such as cylinders and hoses, therewithin. Furthermore, the hose means for supplying hydraulic fluid to the cylinders within the boom are simpler and more compact than known hose arrangements.

We claim:

1. A hydraulic cylinder assembly comprising:

a tubular housing enclosed at one end and open at the other,

supporting means on said housing located intermediate the ends of said housing,

a tubular cylinder rod enclosed at one end and open at the other extending into said housing, supporting means on said rod located near one end thereof,

said housing and rod cooperating to define a retraction chamber and to define an expansion chamber, sealing means carried by said rod to separate said retraction chamber an said expansion chamber, sealing means carried by said housing to seal the other end of said retraction chamber,

a first fluid passage in said rod communicating with said retraction chamber,

a second fluid passage in said rod communicting with said expansion chamber,

and a valve on said cylinder rod for controlling fluid flow to and from said passages.

2. An assembly according to claim 1 wherein said valve is remotely operable.

3. An assembly according to claim 2 wherein said valve is electrically operable.

4. An assembly according to claim 2 including a holding valve mounted on said rod for preventing retract operation of said cylinder assembly.

5. A hydraulic cylinder assembly for use in a telescopic boom comprising at least two relatively movable boom sections comprising:

a housing,

means on said housing located near the middle of said housing to secure the assembly to one of said boom sections,

a cylinder rod extending into said housing,

means on said rod located near an end of said rod to secure the assembly to the other of said boom sections,

said housing and said rod cooperating to define a retraction chamber and a separate expansion chamher,

a remotely operable control valve mounted on said rod and having two fluid ports,

a first passage in said rod interconnecting one port and said expansion chamber,

and a second passage in said rod interconnecting the other port and said retraction chamber.

6. An assembly according to claim 5 including a holding valve mounted on said rod.

7. In a telescopic boom: a plurality of linearly movable boom sections; a plurality of hydraulic cylinder assemblies, one assembly for each movable section, each cylinder assembly comprising:

a housing,

means on said housing intermediate the ends of said housing to secure the assembly to one of said boom sections,

a cylinder rod extending into said housing,

means on said rod near an end thereof to secure the assembly to another of said boom sections,

said housing and said rod cooperating to define a retraction chamber and a separate expansion chamber;

a remotely operable control valve mounted on said rod and having two fluid control ports and two other fluid ports,

a first passage in said rod interconnecting one of said other fluid ports and said expansion chamber,

a second passage in said rod interconnecting the other of said other fluid ports and said retraction chambers;

and two pairs of hoses for said valve, each pair of hoses connected to the fluid control ports of said valve,

one hose in each pair being a fluid supply hose and the other hose in each pair a fluid return hose.

8. A boom according to claim 7 wherein each hose is in a looped arrangement between the valves of adjacent cylinder assemblies.

9. An assembly according to claim 5 wherein said 

1. A hydraulic cylinder assembly comprising: a tubular housing enclosed at one end and open at the other, supporting means on said housing located intermediate the ends of said housing, a tubular cylinder rod enclosed at one end and open at the other extending into said housing, supporting means on said rod located near one end thereof, said housing and rod cooperating to define a retraction chamber and to define an expansion chamber, sealing means carried by said rod to separate said retraction chamber an said expansion chamber, sealing means carried by said cylinder to seal the other end of said retraction chamber, a first fluid passage in said rod communicating with said retraction chamber, a second fluid passage in said rod communicting with said expansion chamber, and a valve on said cylinder rod for controlling fluid flow to and from said passages.
 2. An assembly according to claim 1 wherein said valve is remotely operable.
 3. An assembly according to claim 2 wherein said valve is electrically operable.
 4. An assembly according to claim 2 including a holding valve mounted on said rod for preventing retract operation of said cylinder.
 5. A hydraulic cylinder assembly for use in a telescopic boom comprising at least two relatively movable boom sections comprising: a housing, means on said housing located near the middle of said housing to secure the assembly to one of said boom sections, a cylinder rod extending into said housing, means on said rod located near an end of said rod to secure the assembly to the other of said boom sections, said housing and said rod cooperating to define a retraction chamber and a separate expansion chamber, a remotely operable control valve mounted on said rod and having two fluid ports, a first passage in said rod interconnecting one port and said expansion chamber, and a second passage in said rod interconnecting the other port and said retraction chamber.
 6. An assembly according to claim 5 including a holding valve mounted on said rod.
 7. In a telescopic boom: a plurality of linearly movable boom sections; a plurality of hydraulic cylinder assemblies, one assembly for each movable section, each cylinder assembly comprising: a housing, means on said housing intermediate the ends of said housing to secure the assembly to one of said boom sections, a cylinder rod extending into said housing, means on said rod near an end thereof to secure the assembly to another of said boom sections, said housing and said rod cooperating to define a retraction chamber and a separate expansion chamber; a remotely operable control valve mounted on said rod and having two fluid control ports and two other fluid ports, a first passage in said rod interconnecting one of said other fluid ports and said expansion chamber, a second passage in said rod interconnecting the other of said other fluid ports and said retraction chambers; and two pairs of hoses for said valve, each pair of hoses connected to the fluid control ports of said valve, one hose in each pair being a fluid supply hose and the other hose in each pair a fluid return hose.
 8. A boom according to claim 7 wherein each hose is in a looped arrangement between the valves of adjacent cylinder assemblies.
 9. An assembly according to claim 5 wherein said means on said rod and said means on said housing are trunnion-type mounts. 